A key subsystem in optical systems for a variety of applications is an illumination system which comprises a light source, such as an arc lamp, and several optical components, such as mirrors and lenses, to collect, shape and relay the light from the source to the desired destination. For example, in a projector, light from an arc lamp is collected, made uniform, and made to illuminate an object, such as film or a programmable spatial light modulator, which is then imaged onto a display screen. As another example, in a lithography system, light from the light source is collected, made uniform, shaped into a specific cross-section, and is made to illuminate a photomask having a pattern. The mask is then imaged by a projection lens to a substrate, such as a semiconductor wafer or a display panel, coated with a layer of photosensitive medium.
In all these applications, the intensity of the light illuminating the object must be very uniform spatially. The object, as stated earlier, is, for example, a spatial light modulator (SLM) chip in a projector, or a photomask in a lithography system. Spatial uniformity of a light beam means that the cross-sectional profile of the intensity must be substantially flat. A second important requirement on the illumination system is that its efficiency must be as high as possible so that loss of light is minimized and the smallest possible light source may be used. Alternatively, the highest possible energy may be obtained at the destination surface, such as the display screen or the semiconductor wafer.
Other highly desirable features in an illumination system include compact size and self-luminosity. The importance of a compact size of the illumination system is self-evident—it enables the whole optical system to be compact, and therefore, low-weight, more portable, etc. Self-luminosity of a light source means it is equivalent to an emission surface on which every point behaves effectively as an emission point from which light rays emanate in a specific numerical aperture. Such a characteristic is especially important when the illuminated object must be subsequently imaged with high resolution onto another surface. All of the above desirable features of illumination systems are important in the case of digital projections, lithography systems, and numerous other optical systems.
A self-luminous emission surface is readily obtained by transformation of a high-brightness, point-like light source by use of suitable optical elements. A widely used, high-brightness, point-like light source is a high-pressure, compact, Hg (or Hg—Xe) arc lamp. To increase the amount of collected radiation, and direct it toward the object, such an arc lamp is usually manufactured with a built-in elliptical reflector. An elliptical reflector has two focus points, which I shall call “near focus” and “far focus.” The point-arc of the lamp is situated at a focus point of the elliptical reflective surface, which causes the reflected rays to be directed toward the other focus point of the ellipse, enabling them to enter a beam-uniformization device at a desired numerical aperture. In all such light sources, the requirement of maximum collection efficiency on the one hand and a well-defined numerical aperture on the other hand cannot both be met optimally. This is so because, to maximize light collection, one must use as large a portion of the elliptical surface as possible, whereas to confine the reflected rays to the desired numerical aperture, one must limit the extended arc of the reflector.